Interoperability of magnetic structures for inductive power transfer systems

ABSTRACT

Inductive power transfer apparatus has a first magnetic coupling structure and a second magnetic coupling structure, the structures being adapted to generate and/or receive magnetic flux to thereby transfer power inductively, the first structure comprising a substantially circular coil, and the second magnetic structure comprising an arrangement of at least two coils associated with a magnetically permeable core.

FIELD OF THE INVENTION

This invention relates to inductive power transfer (IPT), and hasrelevance to the magnetic structures, commonly referred to as couplersor pads, that generate and receive magnetic flux for the purposes oftransferring power in a contactless manner.

BACKGROUND

IPT systems provide galvanic and physical isolation to enable safeoperation in wet or dirty environments and consequently the systems arereliable and maintenance free. These hallmarks along with conveniencehave resulted in a large range of applications for IPT systems, such aspowering consumer electronics, Automatic Guided Vehicles and electricvehicles (EVs). Power transfer levels of typical systems vary from 0.5 Wto 50 kW with air gaps between couplers of 1-150 mm. IPT is an idealmethod for recharging EVs because no user intervention is required.Concrete or asphalt may be laid over the ground based coupling structuremaking it extremely durable and vandal resistant unlike any other systemthat uses moving parts. Contactless charging for EVs is normallyachieved inductively through mutual coupling between coils, as in atransformer, however the coupling coefficient (k) is typically in theorder of 0.01-0.5 due to inherently large air gaps.

IPT systems may be loosely grouped as either distributed or lumpedtopologies; the former is suited to applications where continuous poweris required and the latter for cases where power only needs to betransferred at a fixed position. A distributed system comprises aprimary coil laid out in an elongated loop (forming a track) and one ormore secondary coils that couple to a small portion of the trackproviding power to loads. A lumped system is based on discrete primaryand secondary coils and power can only be transferred when the coils areclosely aligned and have sufficient mutual coupling. These systems maybe further broken down into either closely coupled or loosely coupledtypes. Closely coupled systems operate with relatively small air gapsand the user typically has to ‘plug in’ the primary. One example of aclosely coupled system is “charge paddles” which were proposed for earlyelectric vehicles. Loosely coupled systems operate with a large air gapand do not require user intervention; these systems are the subject ofthis specification. The exemplary embodiments are provided in thecontext of recharging EVs and the loosely coupled lumped topology isconsidered the most suitable given vehicles are typically parked inknown fixed locations, for example, parking lots, taxi ranks andgarages. Those skilled in the art to which the invention relates willappreciate that the invention will have relevance to other IPTapplications. In this document the magnetic coupling structures arereferred to as couplers or power pads. The fundamental flux pathsproduced by these typically determines the overall cost and feasibilityof any IPT system.

EV manufacturers consider power levels of 2-7 kW with an operational airgap of 100-250 mm practical, and thus coupler designs capable ofsupplying 7 kW over a 125 mm air gap are considered essential for cars.In addition, the system needs to transfer full power with sufficienthorizontal tolerance (typically more than 100 mm). For Roadway PoweredElectric Vehicles (RPEVs) even larger zones are desirable at higherpower levels and greater separations. Due to the potential for largescale deployment of both stationary charging and RPEV applications,power pads that can work in both applications are desirable. They alsoneed to be as electrically efficient, and cost efficient, as possiblewith the absolute minimum of material use. These requirements make EVcharging a demanding application for IPT.

A typical lumped IPT system comprises three main components, a powersupply, power couplers (often in the form of pads) and a Pick-Up (PU)controller as shown in FIG. 1. The power supply produces a sinusoidalcurrent in the VLF frequency range; it is typically 20 kHz for theexamples referred to herein. This causes resonance between the inductiveTransmitter (Tx.) pad (L₁) and its impedance matched parallelcompensation capacitor (C₁). This approach allows the reactive current(I₁) in L₁ to create a relatively high flux density in the vicinity ofthe transmitter pad thereby boosting power transfer for given switchratings. A typical IPT PU frontend consists of a parallel resonant tankcomprising L₂ and C₂, and a switched mode controller. The voltage acrossthe tank is rectified and a switched mode controller enables it tooperate at a defined quality factor (Q) to boost power transfer andprovide a usable DC output. Parallel compensation is commonly used inbattery charging applications due to its current limitingcharacteristic. Tuning effectively boosts the resonant voltage acrossthe load (QV_(oc)) to match to the battery voltage, while the loadcurrent is fixed by the short circuit current of the Receiver (Rx.) pad.The IPT system will only be able to supply the required power if thecurrent out of the receiver pad is sufficient within the desiredtolerance range. In practice, the turns ratio is adjusted to achievethis. For the example discussed above, the “transmitter” pad is likenedto a primary winding and the “receiver” pad structure which is connectedto, or forms part of, the PU is likened to a secondary winding. However,the invention may also be applicable to bi-directional systems i.e.systems in which the direction of power flow may be reversed.

The power output of an IPT system (P_(out)) is quantified by the opencircuit voltage (V_(oc)) and short circuit current (I_(sc)) of the PUpad as well as the quality factor as shown in (1). This can also bewritten in terms of the VA at the input terminals of the transmitter pad(V_(in)I₁), the transformer coupling coefficient (k) and the operating Qof the receiver circuit as shown.

$\begin{matrix}{P_{out} = {{P_{su}Q} = {{V_{oc}I_{sc}Q} = {{\omega \; I_{1}^{2}\frac{M^{2}}{L_{2}}Q} = {V_{in}I_{2}k^{2}Q}}}}} & (1)\end{matrix}$

The coupling coefficient provides a useful measure for directlycomparing the magnetic properties of different pad topologies and can beeasily determined by taking a few measurements with an LCR meter. Theoperating Q of the PU can be temporality ignored to decouple themagnetic design and the output power; this is then related to thetransmitter driving VA and k². In practice the input voltage to thesupply is limited (e.g. 240V) placing a constraint on the maximumtransmitter VA. Consequently, the output power is highly dependent on kand designs that have maximal k at a given air gap are preferable. Atpresent EV manufacturers are concentrating on small urban vehicles andthese typically have very low ground clearances so that the required airgap between couplers can be as small as 125 mm. A 3D Finite ElementMethod (FEM) software package called JMAG is used to model power padsdriven with a constant current source. There is an implicit increase inthe driving VA as inductances vary with pad movement or parameterchanges such as the coil diameter. In practice, this increase in VA canbe realized by adding series capacitance to the Tx. pad to effectivelylower the inductance seen by the supply however the amount that can bepractically added is limited because it also increases the tuningsensitivity. The peaks in P_(su) and k do not usually occur at the samedesign point. As a result, selecting a pad design that meets theperformance requirements (7 kVA in the examples referred to in thisspecification) typically requires a compromise in coupling and drivingthe pad inductance.

Couplers that are based on circular designs are by far the most commoncoupler topology used in EV charging. A known circular pad constructionis shown in FIG. 2, and is described in our published internationalpatent application W02008/24033, the contents of which are incorporatedherein by reference. Inductively coupled chargers commonly use two powerpads that are circular in shape and may have dimensions of 400 mmdiameter by 25 mm thick for example.

However, to use an inductive charger such as this the vehicle must bepositioned relatively accurately over the charging pad—typically within50 mm of perfect alignment—and the separation (i.e. the verticalclearance) between the power pad on the vehicle and the power pad on theground must be closely controlled. In principle, inductive powertransfer may be accomplished for vertical spacings between 0 mm and 100mm but if the system is set up for 100 mm it will have quite a largereduction in power at 120 mm and may well be inoperable below 50 mm.This state of affairs occurs because both the self inductance and themutual inductance of the power pads vary widely as the distance betweenthe pads changes. Thus at 100 mm the power pad receiver or pick-up mayhave a pick-up voltage of 100 V and a short circuit current of 5.0 A fora power rating of 500 W. If the IPT system electronics operates with a Qfactor of 4, then 2 kW can be transferred to the battery though thereare still difficulties to overcome in producing the power needed at theappropriate battery voltage.

The known power pad construction of FIG. 2 comprises an aluminium case 1containing typically eight ferrite bars 2 and a coil 3. Current in thecoil causes magnetic flux in the ferrite bars and this flux has fluxlines that start on the ferrite bars and propagate to the other end ofthe bar in a path containing the coil that may be thought of as asemi-elliptical shape. The flux lines 4 for a single bar are shown inFIG. 3. The flux lines leave the ferrite in an upward direction andpropagate to the other end of the bar, entering it at right angles. Noflux goes out the back of the pad as the solid aluminium backing plate 1prevents it. In the actual pad the eight bars give a flux pattern shownapproximately in cross section in FIG. 3A. A simulation of the actualflux pattern is shown in FIG. 3B.

From FIG. 3B it can be seen that at the highest point the flux lines areessentially horizontal. Therefore, to get the maximum separationpossible between the primary pad and the secondary pad it would beadvantageous to detect this horizontal flux. However, the horizontalflux is still relatively close to the pad (extending from the padapproximately one quarter of the diameter of the pad) and there is nohorizontal flux at all at the very centre of the power pad. Thus at thevery point where maximum flux density would be ideal—the centre—theactual usable horizontal flux component is zero. References in thisdocument to a circular pad or coupler refer to a coupler having a coilarrangement (which is not necessarily a true circle) that when energiseddirects flux in a manner similar to that shown in FIG. 3B. That is tosay there is little or no horizontal flux along an axis from the centreof a flux generating circular coupler in a direction toward an intendedreceiver coupler.

The induced voltage in the pick-up pad (i.e. the vehicle mounted powerpad) is very separation sensitive—corresponding to the variation inmutual inductance—so that at 120 mm it is reduced by approximately 40%while at 50 mm it is increased by a factor of 2. A reduction in powermeans that the vehicle does not get fully charged in the usual time, butthe more challenging situation is that at smaller separations the powertransferred may be so high that the components of the circuit areoverloaded. Also, as the separation is reduced the self inductance ofthe pick-up coil also changes so that the circuit operates off-frequencyputting extra stress on the power supply. As the separation gets smallerstill this stress on the power supply caused by the non-tuned pick-up onthe primary side cannot be sustained and the system must be shut down.In practice it is feasible to operate with a separation between 40 and100 mm but a larger range is too difficult.

A range of separation from 40 to 100 mm is quite small. If the vehiclehas a relatively high ground clearance then either the power pad on thevehicle has to be lowered or the power pad on the ground has to beraised. Automatic systems for doing this compromise the reliability ofthe charging system. Alternatively the pad on the ground can be on afixed but raised platform but such a pad is a tripping hazard when a caris not being charged and this situation is generally to be avoided in agarage or other location involving vehicles and pedestrians.

OBJECT

It is an object of the invention to provide a flux transmission orreception apparatus that overcomes or ameliorates one or moredisadvantages of the prior art, or which will at least provide thepublic with a useful choice.

SUMMARY OF THE INVENTION

In one aspect the invention broadly provides inductive power transferapparatus comprising a first magnetic coupling structure and a secondmagnetic coupling structure, the structures being adapted to generateand/or receive magnetic flux to thereby transfer power inductively, thefirst structure comprising a substantially circular coil, and the secondmagnetic structure comprising an arrangement of at least two coilsassociated with a magnetically permeable core.

Preferably the coils of the second magnetic structure are arranged toprovide coupling tolerance to relative transverse movement between thefirst and second structures.

Preferably the coils of the second structure extract electrical energyfrom the magnetic field produced by the first structure in differingamounts as the relative transverse position of the structures differs.

Preferably one of the coils of the second structure receives a greaterproportion of a first directional component of a magnetic field producedby the first structure, and another of the coils of the second structurereceives a greater proportion of a second directional component of themagnetic field produced by the first structure.

Apparatus as claimed in claim 4 wherein the components are orthogonal toeach other.

Preferably one component is a vertical component of magnetic flux andthe other component is a horizontal component.

Preferably the components are in quadrature.

Preferably the coils of the second structure are flat coils.

Preferably the coil of the first structure is a flat coil.

Preferably two coils of the second structure are in a side by siderelationship.

Preferably the second structure includes a third coil arranged inspatial quadrature with the at least two coils.

Preferably the second structure includes two coils which overlap eachother.

Preferably the core of the second structure has regions of greater andlesser magnetic reluctance.

Preferably the core of the second structure comprises a lattice.

In another aspect the invention broadly provides an inductive powertransfer system including the apparatus of one of the precedingstatements.

In another aspect the invention broadly provides inductive powertransfer apparatus comprising a first magnetic coupling structure and asecond magnetic coupling structure, the structures being adapted togenerate and/or receive magnetic flux to thereby transfer powerinductively, the first structure comprising a substantially circularcoil, and the second magnetic structure comprising an arrangement of atleast two coils, the at least two coils being located in substantiallythe same plane.

Preferably the at least two coils of the second structure are adjacentto each other.

Preferably the at least two coils of the second structure are in a sideby side relationship.

Alternatively the at least two coils overlap each other.

Preferably the second structure includes a third coil arranged inspatial quadrature with the at least two coils.

Preferably the coils of both structures comprise substantially flatcoils.

In another aspect the invention broadly provides inductive powertransfer apparatus comprising a first magnetic coupling structure and asecond magnetic coupling structure, the structures being adapted togenerate and/or receive magnetic flux to thereby transfer powerinductively, the first structure comprising two coils arrangedsubstantially adjacent to each other and in substantially the sameplane, and the second magnetic structure comprising an arrangement of atleast three coils, two coils being located in substantially the sameplane and adjacent to each other, and a third coil arranged in spatialquadrature with the two coils.

Preferably the coils of both structures comprise substantially flatcoils.

In another aspect the invention broadly provides an IPT system includingthe apparatus of any one of the foregoing statements of invention.

In another aspect the invention provides a method of inductive powertransfer, the method comprising the steps of generating a magnetic fluxusing a first structure having a substantially circular coil, andreceiving the magnetic flux from the first structure using a secondstructure having at least two coils, the at least two coils beinglocated in substantially the same plane.

In another aspect the invention provides a method of inductive powertransfer, the method comprising the steps of generating a magnetic fluxusing a first structure comprising two coils arranged substantiallyadjacent to each other and in substantially the same plane, andreceiving the magnetic flux from the first structure using a secondstructure comprising an arrangement of at least three coils, two coilsbeing located in substantially the same plane and adjacent to eachother, and a third coil arranged in spatial quadrature with the twocoils.

In another aspect the invention broadly provides inductive powertransfer magnetic coupling apparatus comprising a lattice structure of amagnetically permeable material.

Preferably the lattice structure comprises a plurality of linked bars ofmagnetically permeable material.

Preferably the structure comprises a plurality of elongate bars and aplurality of connecting members which connect the bars to provide alattice structure.

Preferably the bars are substantially parallel.

Preferably the bars are arranged in a longitudinal direction and theconnecting members interconnect the bars in a transverse direction.

Preferably the interconnecting members are spaced to provide flux pathsin a plurality of directions through the structure.

Preferably the structure comprises ferrite.

In a further aspect the invention provides an IPT flux pad or couplercomprising one or more energisable coils associated with a magneticallypermeable core having a lattice structure.

Preferably the coil(s) are located on one side of the core.

Preferably the core comprises a plurality of elongate bars and aplurality of connecting members which connect the bars to provide alattice structure.

In another aspect the invention broadly provides inductive powertransfer apparatus comprising a first magnetic coupling structure and asecond magnetic coupling structure, the structures being adapted togenerate and/or receive magnetic flux to thereby transfer powerinductively, the first structure comprising a substantially circularcoil, and the second magnetic structure comprising an arrangement of twoor more coils, at least one of the coils being wound around amagnetically permeable core.

Preferably the two or more coils of the second structure are both woundaround the core.

In one embodiment at least one of the coils of the second magneticstructure comprises a plurality of part-coils.

In one embodiment one of the coils of the second magnetic structureextracts energy from flux travelling in one direction through the core,and another of the coils of the second magnetic structure extractsenergy from flux travelling in a different direction through the core.

In one embodiment the one of the coils of the second magnetic structureextracts energy from flux travelling in a direction at right angles tothe direction of the flux travelling in the other coil.

In one embodiment one of the coils of the second magnetic structurecomprises a substantially flat coil. In another embodiment a pluralityof flat coils are provided in magnetic association with the secondmagnetic structure.

Further aspects of the invention will become apparent from the followingdescription.

DRAWING DESCRIPTION

FIG. 1 is a basic circuit diagram illustrating components of an IPTsystem;

FIG. 2 is a perspective view of part of a known form of IPT powertransfer pad;

FIG. 3 is a diagrammatic elevation in cross section of a part of the padof FIG. 2 showing flux lines;

FIG. 3A is a plan view and elevation of a cross section of the pad ofFIG. 2 showing flux lines;

FIG. 3B is an elevation in cross section of a computer generatedsimulation of the magnetic field (indicated by flux lines) of the pad ofFIG. 2;

FIG. 4 is an elevation in partial cross section of a first embodiment ofan IPT system flux transmitter and receiver arrangement including a padsuch as that shown in FIG. 2;

FIG. 5 is a simplified circuit diagram of a pick-up control circuit foruse with the arrangement of FIG. 4;

FIG. 6 is an elevation of a second embodiment of an IPT system fluxtransmitter and receiver arrangement including a pad such as that shownin FIG. 2;

FIG. 7 is a simplified circuit diagram of a pick-up control circuit foruse with the arrangement of FIG. 6;

FIG. 8 is an elevation of a third embodiment of an IPT system fluxtransmitter and receiver arrangement including a pad such as that shownin FIG. 2;

FIG. 9 is an elevation in partial cross section of a fourth embodimentof an IPT system flux transmitter and receiver arrangement including apad such as that shown in FIG. 2;

FIG. 9( b) is an elevation in partial cross section of a fifthembodiment of an IPT system flux transmitter and receiver arrangementincluding a pad such as that shown in FIG. 2;

FIG. 10 is plan view of a sixth embodiment of an IPT system fluxtransmitter and receiver arrangement including a pad such as that shownin FIG. 2.

FIG. 11 is a simplified model of a DD pad with main flux components(Φ_(a), Φ_(b) and (Φ_(ip), produced by coil a, b and mutual couplingrespectively;

FIG. 12 shows magnetic flux density vectors in the xz plane of a DDQ-DDsystem (a) horizontally aligned and (b) misaligned by 190 mm;

FIG. 13 shows power profiles for quadrature coils of various widths(I₁=23 A);

FIG. 14 shows vertical profiles for different pad combinations (a)P_(su) and (b) k (I₁=23 A at 20 kHz);

FIG. 15 shows power profiles for receiver pads with different orcombined coils on different transmitter pads;

FIG. 16 illustrates 7 kW charge zones for different pad combinations(Q_(max)=6) (a) circular on circular and DDQ on DD, and (b) circular oncircular and DDQ on circular (I₁=23 A at 20 kHz);

FIG. 17 illustrates a bipolar pad seen from the side (top) and fromabove (bottom);

FIG. 18 shows the native VA coupled (Psu=Isc*Voc) with displacement forvarious pad combinations;

FIG. 19 shows the region over which it is possible to achieve 3 kW forthe combinations of FIG. 18;

FIG. 20 shows the actual Q at which the pad operates to deliver thepower for the combinations of FIG. 18;

FIG. 21 shows a Bipolar pad with modified core or back plane usinginterconnecting ferrite blocks that form a lattice to allow better fluxcapture from a circular transmitter pad.

FIG. 22 shows four bipolar pad constructions using different ferriteback planes

FIGS. 23( a)-(d) show voltage, current coupling factor and totaluncompensated VA profiles respectively for x-displacements between aprimary circular pad and various bipolar pad constructions of FIG. 16with similar coil sizes

FIG. 24 (a)-(d) shows voltage, current coupling factor and totaluncompensated VA profiles respectively for y-displacements between aprimary circular pad and various bipolar pad constructions of FIG. 16with similar coil sizes

FIG. 25 (a)-(d) shows voltage, current coupling factor and totaluncompensated VA profiles respectively for x-displacements between aprimary circular pad and two bipolar pad constructions having identicalferrite structure but varying coil size

FIG. 26 (a)-(d) shows voltage, current coupling factor and totaluncompensated VA profiles respectively for y-displacements between aprimary circular pad and two bipolar pad constructions having identicalferrite structure but varying coil size

FIG. 27 shows a plan view from below of a bipolar flux receiver having acore or back plane in the form of a lattice

FIG. 28 shows a plan view from above of a DDQ flux receiver having acore or back plane in the form of a lattice

DESCRIPTION OF ONE OR MORE EMBODIMENTS

Where appropriate throughout the description like reference numerals areused to refer to like features present in the various embodiments.

A first embodiment of a new interoperable arrangement of magneticstructures for an IPT system is shown in FIG. 4. Referring to thatFigure, the system includes a circular transmitter structure and areceiver comprising a core 5 having an E-shaped cross-section(preferably constructed of magnetically-permeable ferrite or a materialhaving similar desirable magnetic properties). The core 5 comprisesthree substantially parallel vertical legs and orthogonal connectionregions connecting adjacent legs to each other. A core of this geometrymay be referred to as a Flat E Core.

The core 5 is provided with a first coil 6 wound around a vertical axis(the central leg of the core 5) and a second coil 3 wound around ahorizontal axis of the core. The pick-up is shown positioned so that thecoil arrangement is situated near the centre of the circular transmittercoil 3. The current flowing in the coil 3 produces anticlockwise andclockwise magnetic fields on opposite sides of the transmitter pad asshown in FIG. 4. As can be seen from FIG. 4, when the receiver structureis centered on the transmitter the magnetic flux travelling through thecentral leg of core 5 induces a current in the coil 6. If the receiverstructure is moved sideways as shown in FIG. 4 (i.e. off-centre), whichwill result in less flux flowing through the central leg, more flux willflow through the horizontal axis of the core 5 and this flux induces acurrent in coil 7. Therefore, this arrangement, in which there are aplurality coils wound around the receiver core to ensure that electricalenergy is extracted from variations in flux due to relative changes inposition between the receiver core and a circular transmitter, allows apick-up to extract power over a wider range of movement i.e. the sytemprovides greater coupling tolerance to realtive transverse movementbetween the coupling structures. Accordingly the system has greatertolerance to misalignment.

FIG. 5 shows a possible tuning and power control circuit for the pick-upconfiguration of FIG. 4, including independent tuning capacitors 8 and9, and rectifiers 10 and 11 for the coils 6 and 7, respectively. Aregulating circuit generally referenced 12 operates in the known way,decoupling the pick-up from the primary coil 3 to supply a constantvoltage to the load 13. This control strategy is described in greaterdetail in U.S. Pat. No. 5,293,308, the contents of which areincorporated herein by reference.

Series tuning capacitors may also be provided if required for each coil6, 7 to ensure that the effective short circuit current and open circuitvoltage outputs of both the first and second coils can be matched,thereby ensuring a power profile that is as even as possible.

FIG. 6 shows a second embodiment of the receiver apparatus for a pick-upaccording to the invention, in which the two coils 6 and 7 of FIG. 4each comprise a set of part-coils (in this embodiment half-coils), 14and 15, and 16 and 17. In this configuration, the vertically directedmagnetic flux from the circular coil 3 of the transmitter pad flowsthrough the central leg of the E core and splits through the connectionportions of the core which are provided either side of the central legand connect the central leg to the remaining two legs of the core. Ascan be seen, axes of the connecting portions are orthogonal to axes ofthe legs of core 5 and the part-coils are provided on the connectionportions of core 5 in this embodiment. Coils 16 and 17 extract energyfrom the flux that flows into the central leg of the core 5 and splitsthrough the connecting portions, so coils 16 and 17 are arranged asshown in FIG. 7 to sum the induced current which will be in a differentdirection in each coil. Coils 12 and 13 extract energy from the fluxthat flows horizontally in one direction through the core 5, so thosecoils are arranged to sum the induced currents. The associated tuningand power control circuit for this embodiment is shown in FIG. 7.

The tuning capacitors in FIG. 7 must all be individually chosen as theyall correspond to different magnetic circuits. Again this wiringarrangement gives the same form of output as indicated in FIG. 5 and canbe extended indefinitely by adding extra track circuits. Also, the Ecore may be extended by adding additional legs and connection portions,with further part-coils. Again, series tuning capacitors can be providedfor the coil sets to match the outputs of the coil sets, and thusprovide a power profile that is as even as possible.

As with the first embodiment, in the embodiment of FIGS. 6 and 7 thearrangement increases the useful range of lateral i.e. transversemovement for an IPT system using a circular pad structure while stillachieving useful power outputs with little change or added cost to theelectronic control circuitry.

Turning to FIG. 8, a further embodiment is shown in which the embodimentof FIGS. 6 and 7 now has the core 5 provided as a flat core structurerather than an E-core. The part-coils 14, 15 and 16, 17 are wound aroundthe core so as to leave a pole areas 20 for receiving (or transmitting)magnetic flux. Operation of this embodiment is the same as describedwith reference to the embodiment of FIGS. 6 and 7.

Turning to FIG. 9, a further embodiment is shown in which the embodimentof FIGS. 6 and 7 now has a core 5 provided as a flat core structurerather than an E-core. The part-coils 14 and 15 are still present tocapture flux that travels in one direction through the core and in thisconfiguration they would be added in phase. An alternative embodiment isto have one larger single coil (a combination of 14 and 15). Thepart-coils 16 and 17 are replaced by a flat coil 21 which captures theflux entering the core 5 in the central part of the core behind the flatcoil 21.

Turning to FIG. 9 b, a further embodiment is shown in which theembodiment of FIGS. 6 and 7 now has a core 5 provided as a flat corestructure rather than an E-core. The part-coils 14 and 15 are stillpresent to capture flux that travels in opposite directions through thecore so in this configuration they would be added out of phase. Thepart-coils 16 and 17 are replaced by two flat coils 22 which are addedout of phase.

FIG. 10 shows a further embodiment of a receiver structure in whichpart-coils 14, 15 of the embodiment of FIGS. 7 and 8 are replaced by asingle central coil 23. The electrical connection is also shown in FIG.10. The part coils 16 and 17 are connected out of phase to produce asecond coil to extract energy from fluxes travelling in the core inopposite directions.

In another embodiment, a new pad topology eliminates unwanted rear fluxpaths by placing two coils above (rather than around) the ferrite stripsas shown in FIG. 11. This form of pad is referred to in this document asa DD pad or structure as described in WO2010090539A1. The ferrite (beingprovided in the form of parallel bars in the embodiment shown thusproviding regions of greater and lesser reluctance in the core tofacilitate desired flux paths) channels the main flux behind the coilsand forces the flux to radiate on one side, resulting in negligibleloss. The ideal DD flux paths are shown in FIG. 11. These paths allowgood coupling to a similar shaped receiver pad because the fundamentalheight is (h_(z)) is proportional to 1/2 of the pad length. A keyfeature to achieving a high coupling factor between two power pads isintra-pad coupling k_(ip). The height of the intra-pad flux (Φ_(ip)) iscontrolled by adjusting the width of the coils in the shaded area ofFIG. 11, to create a “flux pipe” between coil a and b. The fraction offlux (Φ_(ip)) that couples to the receiver pad is mutual flux (Φ_(m))therefore the section of coil forming the flux pipe should be made aslong as possible. Conversely, the remaining length of the coil should beminimized to save copper and lower R_(ac). Doing so results in coilsshaped like a “D” and since there are two such coils placed back toback, the pad is referred to herein as a Double D (DD).

The result of an optimization process was a built pad measuring 770mm×410 mm that uses four strips (as shown in FIG. 11) comprising 6×193cores. The coils were made up of 20 turns of 6.36 mm² Litz wire woundwith a pitch of 6.1 mm resulting in a flux pipe length of 240 mm (43%ferrite coverage).

DD receiver coils can only couple horizontal flux components. As suchthe tolerance of the receiver pad to horizontal offsets in the xdirection can be significantly improved if a second receiver coil isadded. This coil is designed to capture only vertical flux and is inspatial quadrature to the DD. The additional coil requires lengthenedferrite strips to enhance flux capture and the combined structure isreferred to herein as a DDQ receiver. This form of receiver is describedin WO2010090539. The flux plot of FIG. 12( a) shows there is no mutualflux between the DD transmitter and quadrature receiver coils when thepads are aligned (δ_(z) is 125 mm); the DD supplies the full outputpower. However, when the receiver is horizontally offset by 190 mm (asin the flux plot of FIG. 12( b)) the quadrature coil is ideallypositioned to capture the field and able to supply the full outputpower, while the DD is nearing a null in its power profile (this occursat δ_(x)=240 mm).

The DDQ receiver described in WO2010090539 had a quadrature coil whichwas built prior to any optimization, however it is has since beendetermined that a reduced coil width provides better performance. Theoptimization process involves taking power profiles in the x directionfor quadrature coils of various widths. The profiles were done with a125 mm air gap and the results are shown in FIG. 13, the insets show thecoil width range given it has to fit within the DD coils. The highestP_(su) is achieved with a 450 mm wide coil however a width of 410 mm waschosen given the P_(su) is similar but there is an overall reduction inthe length of Litz wire of 1.6m.

The advantages of IPT charging are well recognized and there has beensignificant development of systems by academia and industry. At presentcircular pads are the most common even though they fundamentally offerpoor performance and are not suitable for RPEV applications. Prior toconducting an interoperability investigation via simulation, anexperimental measurement vs. simulation was undertaken to verify themodels containing different pads were accurate (the original built 700mm circular pad was used). The results were found to be in agreement sothe diameter of the coil in the circular pad was adjusted to a δ_(Al) of40 mm and the quadrature coil width was set to 410 mm. The threepossible combinations referred to by ‘receiver on transmitter’ are:circular on circular, DDQ on circular and DDQ on DD.

Prior to conducting any optimisation, some measurements of performancewere made for the combinations referred to immediately above, and alsofor use of a further form of pad structure, referred to herein as aBipolar pad which is shown in FIG. 17. In one embodiment the Bipolar padconsists, from bottom up, of an aluminium plate 37, a dielectric spacer36, a core 34 comprising four rows of ferrite bars 35 (referred toherein as ferrites), two flat substantially coplanar, yet overlappingand ideally but not necessarily “rectangular” shaped coils 32, 33(although in practice these are more oval due to the ease in windingLitz wire) spread out in the lateral direction, and a dielectric cover38. The core 34 acts as a shield so that ideally all flux is channelledthrough the core 4 through the top of the pad. The plate 37 merely actsto a) eliminate and small stray or spurious fields that may be presentabove the core 34 in certain environments, and b) provide additionalstructural strength.

The magnetic structure of the Bipolar pad is designed so that there issubstantially no mutual coupling between either of the coils 32, 33.This allows the coils to be tuned independently at any magnitude orphase without coupling voltage into each other. Each coil can beindependently tuned and regulated without affecting the flux capture andpower transfer of the other coil.

When the two primary coils 32, 33 of the Bipolar pad are placed with anarbitrary overlap with respect to each other, there will be a mutualcoupling between the coils. However for a certain ratio of overlap tocoil width, this mutual coupling is almost zero. The ideal overlaprequired to ensure no mutual coupling exists between each coil is notsimple due the presence of the ferrite but can be determined by simplyfixing one coil and energising this with a predetermined current atfixed frequency (either via a suitable 3D simulator or using a suitableexperimental setup, for example). The open circuit voltage induced inthe second primary coil can then be measured. If the second coil ismoved so as to change the overlap there will be a change in coupledvoltage. When this is minimised (ideally zero) the ideal configurationcan be set.

This magnetic decoupling of coils 32 and 33 allows either of the coilsto be independently tuned and rectified to the output without affectingeach other. As such they can also be turned off (independentlydecoupled) using switches without affecting the flux capture of theother coil.

FIGS. 18-20 show actual measurements of performance of each of circular,DDQ and Biploar pads as receivers for flux generated from a circular padover a 700 mm diameter transmitter with 40A in the pad. The assumedpower requirement is 3 kW.

FIG. 18 shows the native VA coupled (Psu=Isc*Voc) with displacement. Forall but the circular pad, measurements in y and x displacement (in mm)from centre are provided separately as the DDQ and Bipolar pads arepolarized.

FIG. 19 shows the region over which it is possible to achieve 3 kW. Inpractice the actual power out is Po=Q*Psu. The operating Q of the pad isnormally kept below 6, to ensure power can be delivered underuncertainties of tuning, operating frequency and also to ensure lowloss.

The working VA of a pad is Q*P and the quality of the design coil(Q_(L))=300. Worst case losses are VA/Q_(L)=6*3000/300=60 W (2%) at theextremes of maximum power and maximum Q.

If the Q is required to go above 6 it is assumed 3 kW cannot bedelivered, and the power that can be delivered is shown.

FIG. 20 shows the actual Q at which the pad operates to deliver thepower.

Note in all cases, the DDQ and Bipolar pads can deliver the power over awider range than the circular. Here the circular pad can deliver fullpower in all directions with 160 mm displacement.

In the y direction (along the car) the smaller DDQ and Bipolar can alsoachieve this, and the larger DDQ to 180 mm, but will operate even withlarger displacement. In the x direction (across the car), the DDQ andBipolar pads will deliver full power over >320 mm (double thetolerance), and significant power beyond that.

Results for optimised pad arrangements are now discussed with respect toFIGS. 14-16.

Vertical profiles P_(su) and k for air gaps ranging from ˜100 mm-250 mmare shown in FIG. 14( a) and (b) respectively assuming the transmitterand receiver pads are perfectly aligned. The DDQ-DD combination offerssignificantly better power transfer than either of the other two.Although the DDQ receiver pad has not been specifically designed foroperation with a circular transmitter yet is still achieves an almostidentical performance noting the quadrature provides all of the outputpower when the DDQ is centered on the circular pad. The coupling is goodbecause the coils are similar in size, the quadrature coil measures450×391 mm and the circular transmitter is 416 mm. The power output ofthe DDQ is always close to that of a circular on circular design thusthe pad is considered completely interoperable. If circular transmitterpads are present in a charging location, a car with a DDQ receiver canmagnetically couple with this transmitter and if operating at the sametuned frequency can feasibly receive almost an identical full powercharging rate as an EV with a circular coupler specifically designed tooperate with that same transmitter pad.

Circular on circular systems are not polarized but if a polarizedreceiver is used the system becomes polarized and complete 3D powerprofiles are needed for full characterization. A significant number ofsimulations were undertaken to determine the P_(su) profiles shown inFIG. 15 using 20 mm increments in all directions. The vertical air gapfor all simulations is set to 125 mm and l₁ is 23 A at 20 kHz. TheP_(su) profiles of FIG. 15( a)-(e) are arranged such that the firstcolumn shows results for pads excited with a circular transmitter whilethe second column shows the DDQ receiver operating above a DDtransmitter for comparison. FIG. 15( a) shows the profile for a circularon circular system and this clearly indicates the power null that isintrinsic to circular pads. All circular profiles pass through this nullregardless of the P_(su) when the pads are horizontally aligned and thisimposes a fundamental tolerance limit that makes them less ideal forstationary charging without alignment means and unsuitable for RPEV.

The profile for a DDQ excited by a circular transmitter is shown in FIG.15( b). The peak P_(su) is similar to that achieved with a circularreceiver (shown by FIG. 14( a)) however the horizontal tolerance in thex-axis is significantly better due to the contribution from the DDcoils. The thickness of all the pads is the same however the surfacearea of the DDQ is 0.32 m² and the circular pad is 0.38 m² Given theoverall material requirements for a DDQ are marginally greater than thecircular, it would be preferable to have a DDQ receiver over a circularbecause of its far greater tolerance to horizontal misalignment. FIG.15( c) shows the P_(su) for the DD and Q coils separately when excitedwith a DD transmitter, the power output of the DD alone is significantlybetter than that possible using a circular transmitter. Here thequadrature coil contributes significantly to the overall output when thereceiver is misaligned and comes up its maximum output as the DD entersits null. The annotated part of the surface where δ_(x) is large is thepower output of the DD after it has passed through the null. The totalof both the DD and quadrature outputs when excited by a circulartransmitter (FIG. 15( d)) and the DD transmitter (FIG. 15( e)) are alsoshown for comparison. These combinations produce far greater chargezones than possible with circular receivers. The power profiles are bothvery smooth in the x-axis due to the balanced profiles of the individualcoils shown in FIG. 15( b) or 9(c). These profiles demonstrate that theDDQ receiver offers significantly better performance compared to thecircular on circular profile of FIG. 15( a).

Charge zones define the physical operating region where the desiredpower can be delivered given a particular air gap and operational Q. Inthis work a maximum Q of 6 was assumed and the air gap is set to 125 mm.The results of a DDQ receiver pad operating on DD transmitter can becompared with this same pad operating on circular pad in FIG. 16( a) and(b) respectively. Both parts show the charge zone possible if a circularsystem were used for comparison. Notably the physically smaller DDQ-DDpads significantly outperform the circular pads. A DD alone provides acharge zone large enough to enable parking without electronic guidance.Either the quadrature or DD coil can be used to supply the full outputpower in the regions where the DD and quadrature charge zones overlap.The region outside the explicit DD and quadrature charge zones(indicated by DD+Q in FIG. 16( a)) shows the output of either coil isnot enough to provide the desired 7 kW but when both coils are combinedthe power output is ≧7 kW. The small DD ‘zones’ when δ_(x) is very largeshow the DD has passed through its null and is able to supply therequired 7 kW.

The DDQ-DD pads described herein are ideal for RPEV applications becausethe power zone is reasonably smooth in the y-axis. In practice the padswould need to be scaled in size to meet the 20-60 kW required forcharging and propelling a vehicle. To illustrate the concept however,the DD pads can be placed along a road orientated so the width of thepad (shown in FIG. 12) is in the direction of travel (along the y-axis).The DDs investigated here are only 410 mm wide and 7 kW can easily betransferred when the DDQ receiver is offset by 205 mm in the y-axis. Atthis point the DDQ receiver is also effectively offset from an adjacenttransmitter by 205 mm in the y-axis therefore continuous power could beprovided to the EV. Note at that point the power is likely to besignificantly greater than 7 kW due to the contribution from both padsthus permitting the transmitter pads to be positioned in the road with agap between them. This will lower the cost of an RPEV system given fewerpads are needed per km of road regardless of size. Determining the idealpad pitch requires a significant optimization process and is beyond thescope of this paper. The charge zone for a DDQ on a circular pad isshown in FIG. 16( b)—this is a far larger zone than that possible withcircular pads only. The DDQ receiver is considered to be completelyinteroperable with systems based on circular pads and as shown an EVwill fundamentally have more tolerance.

The present invention provides a new polarized coupler (the DDQ) whichproduces a flux path height twice that of a circular pad while alsohaving a single sided flux path. The DD transmitter produces acontinuous flux path and is ideal for RPEV propulsion and charging. TheDDQ pad is also completely interoperable with traditional circular padsand actually offers far greater tolerance than a circular receiver. Asshown, the new DD pad topology makes IPT EV charging significantly morecost effective than conventional designs.

In contrast, existing pads constrain the flux paths. Consider thebipolar pad of FIG. 17. Here the ferrite back plane has bars 25 in asimilar manner to the DD transmitter of FIG. 11. These ferrite stripsare suited to capturing polarized fields extending from the north andsouth poles, however when placed over a circular transmitter, fieldlines are required to extend in various directions to better couple tothe transmitter, but in the receiver these field lines must enter andexit each ferrite strip at right angles, and this increases thereluctance of the path.

Now referring to FIG. 21, a new ferrite back structure is proposed whichincludes strips 45 of overlapping ferrite segments that provides a lowreluctance pathway in all directions suitable for both transmittertypes. As shown in FIG. 22 this can be evaluated against the normalconstruction and variations of this structure where the back plane isreduced relative to the size of the coils and the results are shown inFIGS. 23 and 24. Later comparisons are made with the backplane keptconstant and with coil reduction.

The comparison is made with a primary circular pad constructed to have atotal pad diameter of 420 mm, having a coil inner diameter of 190 mm,coil outer diameter of 286 mm, 8 ferrite spokes, each of length 118 mm.This primary is energized with a 20 A rms current at 20 kHz. Thedistance between the transmitter and receiver pad (z offset) ismaintained at 40 mm for all systems tested, while each of the bipolarreceivers under evaluation are moved laterally in the x and y directions(as per the definitions for movement defined in FIG. 11). When thereceiver is perfectly centered on the transmitter the x, y, z dimensionsin mm are 0,0,40. The bipolar structures have a maximum dimension of300×300 mm. The outer dimension of the coils are 280 mm×280 mm., and areeach made of 5 turns of 4 mm diameter wide Litz, bifilar wound (as suchthey are 40 mm wide). Actual coil dimensions are based on overlapbetween the two coils. In this original model the ferrite structureshows three ferrite strips made up of three standard ferrite bars eachwith dimensions 93 mm long×28 mm wide×16 mm deep. This provides a lowreluctance path to fields extending along the x direction as is expectedif the transmitter is a DD structure.

In later models the ferrite structure uses smaller ferrite bars each 110mm long×15wide×8 mm deep which are overlapped to allow ferriteextensions in both the x and y directions to evaluate any improvement toproviding a low reluctance path to the fields to both the x and ydirections, given a circular transmitter will have fields extendingoutwards from its centre in all directions. The length of the overalllength of the ferrite structure is 280 mm long and width is determinedby the number of ferrite bars chosen. The depth of the ferrite barsnormally is important if there are concerns about saturation. In theinvestigated case the ferrite is never close to saturation and thereforewhat is more important here are the variations shown in the width andlength of the bars. As noted the overall length of the ferrite structurein all cases is similar but the width varies between the evaluatedsystems.

In the structure called P1 (FIG. 22), 26 ferrite blocks are used andplaced under similar sized coils to the original model (except for theoverlap, which is determined for each new model to ensure no mutualcoupling between the two receiver coils). The ferrite structure isidentical in the second model called P1-CS, which is different onlybecause it uses smaller coils (whose outer dimensions are reduced by 10mm to expose 10 mm of ferrite on the x and y faces of the receiver padto encourage flux capture). The final structure called P2 has 16 ferriteblocks and similar coils to that of P1 except for the overlap requiredto ensure no mutual coupling.

FIGS. 23 and 24 compare the simulated operation of the original modelagainst P1 and P2. In FIG. 23 the impact of moving the receiver in the xdirection relative to the circular transmitter is shown, while in FIG.24 movements in the y direction are considered. As noted, all threemodels have essentially identical coil sizes in the receiver (except forsmall differences in overlap to ensure no mutual coupling).

As shown, there is slight asymmetry in the results, and this arisesbecause coil 43 on the bipolar receiver is simulated to be spacedfurther away from the ferrite strips and therefore closer to thecircular transmitter than coil 42. As such, when the receiver isdisplaced to the left of centre alignment coil 43 is found to couplemore power. It should be noted that V1 and V2, and I1 and I2 in FIGS. 23and 24 relate to the voltage and current in coil 42 and coil 43respectively for the bipolar pad. When considering the presentedresults, notably a larger ferrite structure is beneficial particularlyin terms of coupled voltage and over all uncompensated powertransferred. In the original model three bars 28 mm wide are availableto capture flux at the ends (in total 84 mm). In the ferrite structurelabeled P1, 9 thinner bars each 15 mm wide (135 mm in total) captureflux at the ends over a wider area, while in the ferrite structure P2 5thinner bars each 15 mm wide (in total 75 mm) capture over a smallerarea. As noted, with a wider area of flux capture the coupled voltageincreases (when comparing P1 over either the original design with barsor P2), particularly in the centre region, however the captured fluxdrops more rapidly with displacement more than 200 mm in the x directionand 100 mm in the y direction. The total available uncompensated powerin the centre is significantly increased by 43% for P1 over P2 and by26% with P1 over the original bar design. Considering the working rangeof such a small receiver is typically considered to be within xdisplacements of +/−150 mm and y displacement of +/−100 mm, this issignificant.

When considering FIGS. 25 and 26, here two designs are compared withidentical ferrite structures, but with bipolar coils which are varied insize. As shown, in the design labeled “P1-CS” which represented ferritestructure in P1 with coils which have been reduced in size to exposeadditional flux at the extremes for flux capture. When each of the coilsare centered over the transmitter the actual voltage is slightly reduceddue to the smaller coil area, however when offset slightly thesevoltages are similar, in contrast the currents remain largely unaffectedwithin +/−100 mm offset and generally increase (particularly when thebipolar receiver is centered on the transmitter) indicating an improvedcoupling when aligned and an uncompensated power increase of 10%.

A desirable aspect is to ensure improved coupling with tolerance. Assuch, larger coils and a ferrite back plane which can capture flux inall directions is likely to be preferable.

Thus FIGS. 27 and 28 show possible structures which would be suitablefor both a Bipolar and DDQ receiver to ensure they are interoperable forboth polarized and circular structures. The additional ferriteinterconnecting blocks 47 that interconnect bars 45 at selectedlocations enable additional pathways or regions of low reluctance (i.e.transverse pathways and pathways at angles between longitudinal andtransverse directions) for flux when the receiver is displaced laterallyfrom a circular transmitter, while the ferrite strips are best forpolarized transmitters. Thus a lattice core structure is provided.

In FIG. 28, the two side by side double D coils are referenced 48, andthe overlapping quadrature (Q) coils is referenced 49.

Those skilled in the art will appreciate that the structures describedabove may be used bi-directionally. Therefore, although the circular padstructure has been described by way of example as a flux transmitter,the systems may be reversed so that the circular structure is used as areceiver. Furthermore, the new core structures described may be used onflux generating arrangements.

It should be noted that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications may be madewithout departing from the spirit and scope of the invention and withoutdiminishing its attendant advantages. It is therefore intended that suchchanges and modifications be included within the present invention.

1. An inductive power transfer apparatus, the apparatus comprising afirst magnetic coupling structure and a second magnetic couplingstructure, the structures being adapted to generate and/or receivemagnetic flux to thereby transfer power inductively, the first structurecomprising a substantially circular coil, and the second magneticstructure comprising an arrangement of at least two coils associatedwith a magnetically permeable core.
 2. The apparatus as claimed in claim1 wherein the coils of the second magnetic structure are arranged toprovide coupling tolerance to relative transverse movement between thefirst and second structures.
 3. The apparatus as claimed in claim 1wherein the coils of the second structure extract electrical energy fromthe magnetic field produced by the first structure in differing amountsas the relative transverse position of the structures differs.
 4. Theapparatus as claimed in claim 1 wherein one of the coils of the secondstructure receives a greater proportion of a first directional componentof a magnetic field produced by the first structure, and another of thecoils of the second structure receives a greater proportion of a seconddirectional component of the magnetic field produced by the firststructure.
 5. The apparatus as claimed in claim 4 wherein the componentsare orthogonal to each other.
 6. The apparatus as claimed in claim 5wherein one component is a vertical component of magnetic flux and theother component is a horizontal component.
 7. The apparatus as claimedin claim 4 wherein the components are in quadrature.
 8. The apparatus asclaimed in claim 1 wherein the coils of the second structure are flatcoils.
 9. The apparatus as claimed in claim 1 wherein the coil of thefirst structure is a flat coil.
 10. The apparatus as claimed in claim 1wherein two coils of the second structure are in a side by siderelationship.
 11. The apparatus as claimed in claim 10 wherein thesecond structure includes a third coil arranged in spatial quadraturewith the at least two coils.
 12. The apparatus as claimed in claim 1wherein the second structure includes two coils which overlap eachother.
 13. The apparatus as claimed in claim 1 wherein the core of thesecond structure has regions of greater and lesser magnetic reluctance.14. The apparatus as claimed in claim 1 wherein the core of the secondstructure comprises a lattice.
 15. An inductive power transfer systemincluding the apparatus of claim
 1. 16. (canceled)